Depth-dependent coastal Marine Heatwaves: a case study in Shark Bay, Western Australia

Marine Heatwaves (MHWs) are commonly defined as extreme warm weather or climate events and typically have large impacts on the local ecosystems and economy. Coastal seas that connect the open ocean and land are highly impacted by local terrestrial weather and climate systems. Distinct geographical features (e.g. water depth and bottom topography) of each coastal sea may locally contribute to the spatiotemporal pattern and associated drivers of coastal MHWs. To unravel this undetermined contribution, we choose the Shark Bay (Western Australia) as a case study domain. It is a semi-enclosed bay adjacent to the warm Leeuwin Current with in-bay water depth varying around 0 to 25m and out-bay depth from 25m down to 200m in the selected study area. Thus, the contribution of air-sea heat flux, advection, and other oceanic processes can be quantitatively evaluated by applying the mixed layer heat budget analysis based on a 0.1-degree model reanalysis dataset, Bluelink ReANalysis (BRAN) 2020. Additionally, three high-resolution satellite sea surface temperature (SST) products are used to identify, visualize, and analyze the spatiotemporal patterns of MHWs in Shark Bay. The spatial maps of MHW mean duration, mean cumulative intensity and event frequency exhibit a highly consistent pattern with large differences between metrics in shallow and deep areas. Mixed layer heat budget analysis within a month before each corresponding peak day of three selected major events, to some extent, confirms that this distinct spatial pattern is partially due to the constrained contribution of the entrainment processes below the mixed layer in shallow areas. The entrainment processes that are closely related to the mixed layer depth change may warm the surface layer during mixed layer shoaling by excluding less warm water below the mixed layer. This is not the case in very shallow regions. Interestingly, slightly different from what was previously assumed, the in-bay areas, instead of being warmed by the horizontal advection when the out-bay areas are warmed by the anomalous warming Leeuwin current, are slightly cooled by a constrained net cooling effect. We found that coastal MHW events in shallow areas are typically frequent but less intense if they occur independently under the typical net cooling effects of horizontal advection. Whereas coastal MHWs in deep areas are less frequent, but more intense and prolonged when concurrent with anomalous warm water advection. The shallowest in-bay areas that are not included in the heat budget analysis are outside the influence area of the net cooling effects. Thus, these areas may be intrinsically embedded with frequently fast warming effects of the net heat flux. By using the 90th percentile definition, these frequent warming are defined as MHWs, but the regularity of historical events may not lead to catastrophic impacts regarding the shorter duration and smaller cumulative intensity of an individual event. We then suggest that a global assessment of the net cooling effects of horizontal advection is necessary, to identify qualified coastal areas associated with higher resistance to sudden and prolonged ocean warming.